1. Field of the Invention
The invention relates to a method of so-called xe2x80x9cultrasonicxe2x80x9d peening employing a mist of microbeads inside a chamber and, more particularly, relates to a method for peening parts on a wheel, such as the aerofoils of turbo machine blades on a rotor. The invention also relates to a peening machine essential in implementing the present method.
2. Summary of the Prior Art
The term xe2x80x9cwheelxe2x80x9d is to be understood as meaning an object with an overall shape that is axis symmetric about a geometric axis, it being possible for this object to be rotated about this axis.
To improve the fatigue strength of mechanical parts, it is known practice for their surface to be peened by blasting with microbeads. This technique is widely used in aeronautics. Peening involves impacting the surface of the part at a low angle of incidence with respect to the perpendicular to this surface. Thus, with sufficient kinetic energy, the microbeads cause permanent surface compression over a shallow depth. This surfaces compression inhibits the initiation and propagation of cracks on the surface of the part, which improves its fatigue strength. Typically, this angle of incidence has to be smaller than 30xc2x0 for the impacts to be able to transmit sufficient energy from the bead to the impacted surface. The exposure of the part to peening passes through an optimum level which gives this part the best strength. Insufficient peening does not give the anticipated strength, but it is still possible to achieve the optimum by performing additional peening. By contrast, excessive peening causes surface damage to the part with a drop in its strength. This damage cannot be recovered and the part has to be scrapped.
Peening is customarily performed using nozzles supplied both with compressed gas and with microbeads. The compressed gas propels the microbeads towards the parts. This method of peening has two disadvantages per se:
the peening parameters are unstable and the peening machine has to be checked and adjusted frequently in order to achieve peening that is close to the optimum level,
the peening method has to be performed inside a booth which is large enough to allow the parts and the peening nozzles to be handled.
It is known practice in the aerospace industry for the flanks of the blades of aircraft turbine engine rotors to be peened. When the blades are manufactured separately, they each comprise a thin aerofoil and a root for holding the blade. To peen the thin aerofoil, the blade is held by the root and peening is performed with two nozzles pointing towards one another on each side of the acrofoil. One of the nozzles peens one Hank of the acrofoil and the other nozzle peens the other flank of the acrofoil. The two nozzles sweep across the flanks of the aerofoils and are moved with the best possible synchronisation so that peening progresses symmetrically.
When this symmetry is not achieved, stress peaks appear under the most peened flank. These stress peaks reduce the ability of the blade to withstand the loadings and lead to blade deformation. Peening symmetry is tricky to achieve and to maintain because of the spread and drift in the peening parameters inherent to this nozzle peening method. It will be understood that close to optimum blade peening is a lengthy and expensive operation because it has to be performed blade by blade with great precision.
Attempts have also been made at peening the blades of bladed wheels directly in the manner described above. These wheel and blade assemblies are made as a single piece, the acrofoils of the blades projecting from the wheel. Peening has to be achieved on the flanks of the aerofoils and the surfaces of the wheel known as the xe2x80x9ctinter-blade spacexe2x80x9d, ie the surfaces located between two side-by-side aerofoils and possibly around these aerofoils. The blades can be mounted on the wheel or alternatively the aerofoils may be integral with the wheel.
Peening on bladed wheels is performed blade by blade as follows:
the two flanks of each blade are peened in synchronism using two deflected-jet nozzles entering the inter-blade spaces, that is to say the spaces located between two side-by-side aerofoils, said nozzles each comprising a reflector for deflecting the stream of microbeads through 90xc2x0 and directing it normally onto the flanks of the blades.
then using a direct-jet nozzle to peen the inter-blade surface of the wheel.
One disadvantage with the above is the inevitable overlap of the peening of the flanks of the aerofoils and the peening of the inter-blade surface in the transition zone between said flanks and said inter-blade surface. It will therefore be understood that this transition zone is peened twice.
One major disadvantage of the above is that it is impossible to use the method when the inter-blade space is too narrow for the peening nozzles to gain access, which is often the case with present-day bladed wheels.
International patent application WO 95/17994, particularly designating the United States of America, discloses an ultrasonic peening machine employing a titanium bowl. The bottom of this titanium bowl is vibrated by a sonotrode associated with a magnetostriction vibrator. The parts that are to be peened are suspended from a lid placed over the opening of the bowl. The entire bowl is vibrated and, along with the lid. constitutes a peening chamber inside which a mist of microbeads is thus sustained. This peening chamber does not allow the peening of thin parts, such as the aerofoils of bladed wheels, as the mist of microbeads is heterogeneous, not least because the distribution of vibrational energy is highly complex and has antinodes and nodes. In addition, a very large titanium bowl is needed to form a peening chamber capable of peening a complete bladed wheel. Such a bowl is very expensive and requires a powerful vibration generator.
In French patent 2 689 431, a method for peening the teeth of a pinion is disclosed. The pinion is rotated past a sonotrode, the teeth of the pinion passing the sonotrode in turn. The sonotrode is surrounded by a curtain of spring-loaded rods, which come into contact with the teeth and with the pinion to form a sealed chamber around the sonotrode. The deformable edges of the chamber follow the profile of the teeth and of the pinion. The method of FR 2 689 431 cannot be applied to a bladed wheel because:
the peening of the flanks of the aerofoils would be highly asymmetric,
the rods would not automatically be able to follow the flanks of the aerofoils which are too closely spaced and close to the radial position.
In addition, optimum peening is difficult to achieve because peening would have to be halted precisely in order to treat the entire periphery of the wheel without subjecting that part of the wheel which was exposed to the peening first to any additional peening.
A first problem to be solved is that of peening the flanks of the aerofoils of a bladed wheel when said flanks are too close together for peening nozzles to gain access.
A second problem is that of peening the flanks of the aerotbils and the inter-blade surfaces of the wheel without peening the transition zones between said flanks and said inter-blade surfaces twice.
A third problem is that of speeding up the peening of the aerofoils and of the inter-blade surfaces of a bladed wheel.
A fourth problem is that of improving the symmetry of the peening on the opposite flanks of the aerofoils.
The invention proposes a method for ultrasonic peening of parts on a wheel comprising an annular surface centered on the geometric axis of rotation of the wheel, the method including the steps of:
a) providing the parts on said annular surface of the wheel aligned on a geometric circle centered on the geometric axis of rotation so that the parts of the wheel define a geometric surface of revolution,
b) providing at least three chambers with respective openings having lateral edges,
c) providing said wheel simultaneously above the openings of said at least three chambers so that at least some of the said parts extend into said chambers through said openings and said lateral edges are arranged on each side of said parts on said wheel.
d) providing said lateral edges with a limited clearance E1 from said annular surface
e) providing said openings with two shaped edges facing one another so that said shaped edges face the geometric surface of rotation with limited clearance E2,
f) providing each chamber adjacent its neighbouring chamber with a shaped edge between these chambers,
g) providing at least one chamber with a vibration surface so that said chamber is an active chamber whilst at least two other chambers are passive chambers,
h) providing each active chamber between two other chambers either side,
i) providing at least one active chamber with a plurality of microbeads whereby said vibration surface energises said plurality of microbeads to form a mist of microbeads in the said active chamber,
j) rotating said wheel about its geometric axis of rotation in order that said parts on said wheel pass successively through said chambers whereby said mist of microbeads provides peening of said parts on said wheel,
k) supplying microbeads to said at least one active chamber and recovering any of said microbeads which are carried into and fall in any of said passive chambers.
It will be understood that the parts may be removable on the wheel or alternatively may be incorporated into the wheel through continuity of the material from which they are made. Whichever the case may be, the parts each pass in turn into each chamber under the effect of the rotation of the wheel, which allows all of them to be peened. It will be readily understood that an active chamber never opens via a shaped edge directly to the outside but always opens via at least one passive chamber, only a passive chamber being able to open to the outside via a shaped edge. It will be understood that there is only a limited clearance between the lateral edges and the shaped edges and so these edges collaborate to seal the chamber against microbeads with respect to the wheel and to the parts, said sealing being a contactless seal. In effect, the lateral edges close the chambers on the annular surface of the wheel, said annular surface thus progressing along the lateral edges with a clearance E1, and said parts progressing between the lateral edges when the wheel rotates about its geometric axis of rotation. Likewise, the shaped edges close the chambers on the envelope surface, the parts passing transversely past the shaped edges with a limited clearance E2. This seal needs to be sufficient for the mist of the microbeads to remain concentrated in a small volume so that the energy behind it is not dispersed excessively.
It will be understood that microbeads gradually escape from an active chamber by passing through the spaces between parts, that is to say between two parts, when said spaces between parts pass a shaped edge. As an active chamber is adjacent, at each of its shaped edges, to another chamber, these microbeads arrive in the adjacent chambers. There are two scenarios: if this adjacent chamber is a passive chamber, the microbeads which have entered it no longer receive any energy from a vibrating surface and so quickly drop to the bottom of said passive chamber as the energy driving them is used up; if this adjacent chamber is an active chamber, microbeads will once again escape through the spaces between parts and will enter the two adjacent chambers, and so on, until they reach a passive chamber in which they will use up their energy and drop to the bottom. It will thus be understood that during peening, a stream of microbeads from the chambers to the passive chambers is created, this stream passing mainly through the spaces between parts, the microbeads accumulating in the passive chambers being recovered and advantageously reintroduced into the active chambers to supply these with microbeads.
It has been found that the mist of microbeads enters the narrow spaces between the aerofoils very well, right down to the inter-blade surface of the wheel, which makes it possible to completely peen the flanks of the aerofoils and solves the first problem of peening nozzle access. The inter-blade surfaces are peened at the same time as the flanks of the aerofoils. Thus, the transition zones between the flanks of the aerofoils and the inter-blade surfaces are peened only once which solves the second problem of repeat peening.
Typically, the conventional time taken to peen a set of 75 aerofoils in a booth is as much as 24 hours given the numerous intermediate handling operations required between each aerofoil. With the present method, this time is brought down to 90 minutes by eliminating these intermediate operations which solves the third problem of productivity.
In practice, the clearance E1 between the lateral edges and the annular surface is less than the diameter of the microbeads, which completely prevent the microbeads from passing through this clearance E1 and thus avoids the need to use additional means to recover microbeads which could otherwise have escaped through this clearance E1.
Advantageously, the clearance E2 between the shaped end and the envelope surface is at most equal to twice the diameter of the microbeads. This makes it possible to reduce the amount of microbeads passing from one chamber to another. This clearance E2 can also be given a value smaller than the diameter of the microbeads, which would completely prevent said microbeads from passing through this clearance E2 from one chamber to another, but this very small clearance E2 still obviously has no effect on microbeads passing through the spaces between parts from one chamber to another.
Advantageously, the circumferential width L1 of the chambers measured between the shaped edges is at least equal to three times the circumferential distance L2 between two consecutive parts, L1 and L2 being arc lengths along the two metric circles formed by the parts. In other words, a chamber may contain up to four parts simultaneously. In the ease of the active chambers, an arrangement such as this makes it possible for a mass of mist of microbeads to be sustained in this chamber, which mass is greater than the mass capable of escaping through a space between parts as it travels past a shaped edge, regulating said mass. In the case of the passive chambers, an arrangement such as this enlarges the chamber, encourages the microbeads to use up their energy and thus makes it possible to reduce the proportion of microbeads capable of escaping from the chamber. The effects produced are, however, improved when the ratio L1/L2 is larger, for example at least equal to five or ten.
Advantageously, the wheel makes at least N=5 revolutions during peening. With an arrangement such as this, each part receives, on each revolution of the wheel, just a fraction equal to 1/N of the total peening that is to be performed, which means that the parts can be peened in a uniform way close to the optimum. In fact it will be understood that the parts normally pass through an active chamber N times, but some of them will pass through the chamber N+1 or Nxe2x88x921 times depending on the way in which the least revolution is performed, this difference 1/N becoming negligible when N is large.
An arrangement such as this is particularly advantageous in the case of thin parts such as turbo machine blade aerofoils. Specifically, when a blade enters the vibrating chamber, its flank facing in the direction of rotation of the wheel comes to face the vibrating surface and its peening will take precedence over that of the opposite flank, whilst the converse occurs when this same aerofoil emerges from the active chamber an instant later. Thus, the way in which peening progresses on the opposite flanks of the blade is asymmetric as said aerofoil passes through the vibrating chamber, this asymmetry being absorbed when the acrofoil emerges from the vibrating chamber, this asymmetry therefore being divided by N and consequently becoming negligible, which solves the fourth problem defined above.
Advantageously, the active chambers and the vibrating surfaces are symmetric with respect to a vertical geometric plane P containing the geometric axis of rotation. With an arrangement such as this, the mist of microbeads obtained in the active chambers is symmetric with respect of this plane P, so that the rear and front flanks of the aerofoils in the mist of microbeads follow equivalent peening cycles, improving the overall symmetry of the peening performed on the flanks of the acrofoils.
The present invention also provides a peening machine including a spindle for holding and rotating a wheel about a geometric axis of rotation, the peening machine comprising a plurality of chambers including at least one active chamber having a vibrating surface for sustaining a mist of microbeads in said active chamber, the chambers each comprising an opening facing toward the geometric axis of rotation, each opening being delimited by two lateral edges facing each others one of the lateral edges of each opening being positioned on a first arc of a geometric circle centered on the geometric axis of rotation, while the other lateral edge of each opening is positioned on a second arc of a geometric circle, centered on the geometric axis or rotation, each opening also comprising two shaped edges which are identical and arranged on a geometric circle centered on the geometric axis of rotation, the said chambers each being adjacent to the next chamber with a shaped edge therebetween, each active chamber being located between two other chambers and means for supplying each said active chamber with microbeads and means for removing the microbeads from chambers other than each active chamber.
Advantageously, the means for supplying the active chambers with microbeads and the means for removing the microbeads from the passive chambers consist of thalwegs comprising high points and low points, said low points being in the active chambers and arriving at the vibrating surfaces, said high points being in the passive chambers. It will be understood that the thalwegs drain the microbeads which have dropped to the bottom of the passive chamber away under gravity to return them to the vibrating surfaces in the active chambers. These thalwegs pass through the lateral walls of the chambers by passing through tunnels.
Advantageously, the chambers are removable. Such chambers being of simple construction, for example made of plexiglas panels, this arrangement allows the peening machine to be adapted very simply to suit wheels and parts of different shapes and sizes.